The proposed research will focus on the means by which messenger RNA regulates protein synthesis in a differentiating cell. The myoblast (embryonic muscle cell) will be the main model for this investigation for the reason that the differentiation of this cell can be most carefully and precisely controlled in the laboratory; and because, upon fusion of these cells to make muscle, myosin, an easily recognized and characteristic protein in muscle, is produced in large quantities. Messenger RNA, transcribed from the DNA genes for muscle (the myosin message), is somehow controlled by the cell, which may regulate its stability, rate of synthesis, or translation into protein, as compared with other non-specific messages. How this is done can be ascertained by techniques we have recently developed for use with the HeLa (cancer) cell. These techniques depend on the fact that messenger RNA has a unique nucleotide sequence (poly A) which allows its isolation from the more prevalent kinds of RNA by annealing it to a complementary nucleotide sequence (oligo dT polymers). Once isolated, the rates of formation and decay of myosin message vs. non-specific messages can be investigated for differential stabilities or rates of synthesis. These rates can then be usefully compared with the kinetics of synthesis of myosin protein vs. total protein synthesis. Many investigators have found that actinomycin, an inhibitor of RNA transcription, is ineffective in stopping synthesis of differentiated proteins, whereas in the relatively undifferentiated HeLa cell, protein synthesis ceases almost immediately in the presence of this drug. We have found that this inhibition of protein synthesis occurs much faster than could be accounted for by inhibition of RNA synthesis, apparently because actinomycin inhibits translation of mRNA into protein. This research is designed to elucidate mechanisms regulating cell growth and differentiation, as well as those aberrant mechanisms which supercede them in cancer cells.